Earthing of Isolation Transformers
On any electrical diagram of control equipment, you can find a large number of associated grounds. Many of the connections are obvious, such as different control cabinets, different power lines, etc. But there is often confusion about the many ground connections associated with transformers. Transformers exist in almost every power supply system, as well as in most control systems. They are often used with three-phase mains to provide 240V or 120V single-phase for use with loads such as heaters, DC power supplies, and other connected equipment. A transformer is a complex device and often involves specific connection elements that are not obvious, but, fortunately, are often included in circuits.
This article will cover the issue of grounding transformers, as well as several common ground connection configurations. Each piece of equipment is different, so this will not cover every situation, but this material should provide clarity and rationale as to why there are so many groundings in circuits.
Ground connections provide a redundant protective conduit intertwined between each piece of equipment and metal objects that may come into contact with electricity. If, due to a malfunction or improper connection, the live wire looses and touches any nearby metal, a low resistance, free-flowing circuit will be created. The resulting large current should ideally trip the breaker or fuse almost immediately.
Since a fault can be caused by almost any metal object near a live wire, it seems logical to examine every point in and around every circuit to make sure it is properly grounded.
When connecting transformers, there are almost always three ground connection points, with the rare exception of two, as will be discussed later. These three points are primary winding, housing and secondary winding.
Primary grounding
Most often, the primary winding of the transformer in the control system is powered from a single-phase or three-phase high voltage source. When properly connected, this incoming power line was already connected to earth when it entered the installation or control cabinet.
Therefore, it is not uncommon to see a ground symbol affixed to the circuit right on the input primary, but be sure to trace it back to the source to make sure it is actually already grounded.
Grounding the transformer housing
Almost every chassis transformer chassis is made of metal, although some are small plastic ones. If the case is metal, there should be no voltage on it. The purpose of the transformer is to retain all the electrical conductivity contained in the wires around the coils - no electricity should flow into the iron core or metal chassis surrounding the coils.
However, in the event of a wire breakage malfunction, this chassis may be energized, and if this occurs, it connects to an electrical path of near-zero resistance. It is very important that the metal housing of the transformer is firmly connected to earth and not just to the primary wiring.
Secondary grounding
One of the advantages of a transformer is that it provides isolation between the incoming and outgoing lines, even if there is no voltage change. The core does not conduct electricity, so although it generates a corresponding voltage on the output side, this output wire will never be physically connected to the main power input.
From a grounding point of view, this means that the secondary output must be grounded as it is the other isolated part of the circuit. Any wiring faults downstream of the transformer must be able to briefly re-enter the circuit to disable this circuit protector. As with any power supply, this is the neutral wire that is connected to ground.
Standard Transformer Grounding Configurations
Transformers come in a variety of wiring configurations. Although not all of them are covered in this article, this material will help you understand the most common: single-exit and double-exit.
If the transformer has one single voltage output, which can be either 240 V or 120 V, in order to further provide power for loads of the 24 V type, which is typical for many control devices.
In any case, one of the two wires must be grounded. This wire becomes the neutral wire. If there is only one output voltage, it does not matter which wire is neutral and grounded, although a specific wire is sometimes preferred to maintain phase rotation between devices.
With single output transformers in which sometimes, but very rarely, there will be a secondary winding that is not grounded. It is installed only to prevent the propagation of electromagnetic noise through the ground near the output. It is usually only connected this way when testing and troubleshooting equipment using ground test devices such as oscilloscopes.
It is often the case that a higher voltage transformer will produce two different voltage levels. Most often this is a 120V / 240V output. Three lines will come out of the secondary winding - one at each end of the winding and one from the center, called the center tap.
There are two different strategies for using this central branch. First, if this center tap is connected to earth, it is called neutral. If the load device is connected to two external wires, it will receive 240V. If the load is connected between one of the external sides and this center neutral, it will only receive 120V. Thus, there are two ways to get 120V (line to neutral) and one way get 240V (line to line). This is a normal configuration for household wiring as it allows the load circuits to be effectively controlled.
The second strategy is to ground one of the outer wires, making it neutral. The load connected between the center tap and external neutral will receive 120 V. If the load is connected from output to output, it will receive 240 V. Thus, the voltage outputs are the same, but both the lower and higher voltages are limited to one output wire, which ensures efficient control ... The second strategy, in which one of the outer wires is connected to ground, making it neutral, is shown in the following figure. 120 V is provided from the center to neutral, and 240 V is supplied to neutral from the outside.
Transformers are simple devices, but their connections can be confusing at times. Often they just plug into the control circuit with the hope of doing it right, but without a clear understanding of how it works. For most industrial control systems, it would be normal to provide a reliable earth connection on the primary side as well as on the case itself. The secondary winding must also be grounded, but the most appropriate option for the specific equipment must be selected.
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